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The immune defense system is a body-wide network of organs, tissues, cells, and protein substances that work together to defend the body against attacks by "foreign" invaders. Those invaders are primarily germs—tiny, infection-causing organisms such as bacteria and viruses, parasites and fungi. (See box on Germs)
The immune system is amazingly complex. It can recognize millions of different enemies, and it can enlist specialized cells and secretions to seek out and destroy each of them. (Substances recognized as foreign that provoke an immune response are called antigens.)
The organs of the immune system are known as lymphoid organs because they are home to lymphocytes, small white blood cells that are key components of the immune defenses. Bone marrow is soft tissue in the hollow center of bones, and it is the original source of all blood cells. The thymus is an organ that lies behind the breastbone; that is where some lymphocytes mature. The spleen, located in the upper left of the abdomen, serves as headquarters for many immune system activities.
|T Y P E S O F W H I T E B L O O D C E L L S|
|Immune cells, once alerted to danger, undergo important changes. They begin to produce powerful chemicals that allow the cells to grow and multiply, and to attract and direct their fellow cells.|
To work well, most immune cells need the help of other immune cells. Sometimes immune cells communicate with one another by direct physical contact, sometimes by releasing chemical messengers.
Each type of immune cell has its special role. B cells work chiefly by making plasma cells that secrete antibodies. Antibodies are large molecules that attach to invading germs (and other foreign particles) and mark them for destruction.
T cells contribute to the immune defenses in two major ways. Helper T cells and cytotoxic T cells secrete powerful chemicals (cytokines) that allow them to control the immune responses, including the work of B cells. Natural killer cells directly attack cells that have been infected by viruses.
Phagocytes are large white blood cells that act as scavengers. They roam through the body, engulfing germs and destroying them. Neutrophils and monocytes are phagocytes that contain bags of potent chemicals that help destroy the germs they engulf.
| Antibodies are blood proteins known as immunoglobulins. They are produced by B cells. Different types, or classes, of immunoglobulins play different roles in immune defenses. As an immune response unfolds, B cells gradually switch from making one type of immunoglobulin to another. |
- Immunoglobulin M (IgM) is the first to respond to an invading germ. IgM antibodies tend to stay in the bloodstream, where they aid in killing bacteria.
- Immunoglobulin G (IgG) follows on the heels of IgM. It is the main immunoglobulin working in the blood and tissues. IgG antibodies coat germs so that immune cells have an easier time of engulfing them.
- Immunoglobulin A (IgA) is produced along surface linings of the body and secreted in body fluids such as tears, saliva, and mucus, where it protects the entrances to the body—mouth, nose, lungs, and intestines. It is also present in breast milk and provides important protection against bacteria in the intestines of newborns.
- Immunoglobulin E (IgE) which is normally present only in trace amounts, is an important component of allergic reactions.
Another important component of the immune defenses is the complement system. The complement system is composed of a series of more than 20 blood proteins that, when activated, work closely together in a step-wise fashion. Complement helps antibodies and phagocytes destroy bacteria and acts as a signal for recruiting phagocytes to sites of infections.
Although the immune system is designed to recognize and attack foreign invaders, its recognition program sometimes breaks down. Then the body begins to make T cells and antibodies directed against its own cells and organs. These misguided T cells and these autoantibodies, as they are known, contribute to "autoimmune" diseases. For instance, T cells that attack pancreatic islet cells contribute to diabetes, while certain autoantibodies are common in persons with rheumatoid arthritis.
Lymphocytes can travel throughout the body, using the blood vessels or a system of lymphatic vessels. The lymphatic vessels carry a clear fluid known as lymph. Scattered along the lymphatic vessels are small, bean-shaped lymph nodes, where immune cells gather and interact.
Clumps of lymphoid tissue are found in many parts of the body, especially in the linings of the digestive tract and the airways and lungs—areas that protect gateways into the body. These tissues include the tonsils, adenoids, and appendix.
The immune system makes use of many types of white blood cells. These include two main kinds of lymphocytes, T lymphocytes (T cells) and B lymphocytes (B cells); and a class of cytotoxic lymphocytes called natural killer (NK) cells. Additionally, there are large white blood cells known as phagocytes (neutrophil and monocyte).
G E N E S A N D P I
In the past few years, scientists have succeeded in identifying the genes that are responsible for many PI diseases. These include X-Linked Agammaglobulinemia, X-linked Hyper-IgM Syndrome, Wiskott-Aldrich Syndrome, Ataxia Telangiectasia, four forms of Chronic Granulomatous Disease, and several forms of SCID. The search for other genes that cause PI is under way and more are being discovered.
Sometimes the same, or nearly the same, symptoms can be the product of different defective genes on different chromosomes. For example, SCID can be caused by mutations in different genes. One genetic defect blocks activation of B cells and T cells. Another genetic defect prevents immune cells from getting rid of toxic chemicals. In every case, however, the end result is the same: major immune defenses are non-functional.
Once researchers have identified the defective gene, they try to find out what it normally does, what protein it makes, and how that protein contributes to the immune response. Some proteins, for example, relay signals that tell immune cells to multiply and mature. Other proteins help the immune system to eliminate excess or unwanted cells.
The next step is to ascertain what happens when the protein is missing or distorted and how the faulty protein causes disease.
Learning about a disease-causing gene and its protein product raises the exciting prospect of finding a cure for the disease.